Electronic Pairing in Exotic Superconductors

Investigations of rare earth, Aactinide, organic and oxide compounds have yielded several new classes of exotic superconductors. These include magnetically ordered superconductors, A15 superconductors, buckyball superconductors, heavy-electron superconductors, organic superconductors and high-Tc oxide superconductors. These materials have properties significantly different from those of conventional superconductors such as Al and Zn, which are described well by the Bardeen-Cooper-Schrieffer model of superconductivity. We carefully distinguish between the BCS model and the more general BCS theory. In the BCS theory superconductivity arises, loosely speaking, from electron pairs that behave essentially as bosons and undergo macroscopic condensation to the lowest energy state at the critical temperature Tc The BCS model, presented in 1957, further specifies that the pairing is mediated by exchange of quantized lattice vibrations (phonons) between the electrons, yielding pairs with zero spin S (spin singlet) and zero angular momentum L (s wave). This model is but one example of the BCS pairing theory; another describes the superfluid state of 3He, where the fermionic 3He atoms form p-wave (L = 1) spin-triplet (S = 1) pairs held together by the exchange of magnetic excitations of the surrounding atomic sea.